Coating compositions, particularly polyurethane coatings produced by reacting isocyanate-functional resins with hydroxyl-functional polyol resins, are well known per se. These coatings are distinguished, as is known, by a range of outstanding properties, including high durable adhesiveness and chemical resistance. On account of their good glamour properties they are outstandingly suitable for automotive applications and automotive refinish applications, and also for industrial applications. In comparison to aqueous compositions they exhibit substantially better drying times and an enhanced corrosion control. With these coatings, however, owing to the formation of hydrogen bonds and owing to the molecular weight of the main polymer chain, it is the polyol component that makes the greatest contribution to the overall viscosity of the coating, thereby restricting the freedom to incorporate other substances at formulation and to meet the specifications in terms of application solids content. The reduction in the molecular weight of the polyol leads ultimately to slow network construction, unless considerable quantities of energy are introduced.
Through the use of greater amounts of solvent it is indeed possible to reduce the overall viscosity of the coating and so to accelerate network construction, but using large amounts of solvent is environmentally objectionable. In particular, coating compositions with high proportions of volatile organic compounds, especially solvents (VOC), do not meet the provisions of guidelines in which the maximum VOC content is laid down. In this context, in the future, it will be necessary to realize VOC contents of not more than 250 g/l.
The properties of high-solids, polyol-containing polyurethane coatings can also be enhanced through the use of reactive diluents. One application of this technique has been demonstrated by U.S. Pat. No. 5,214,086. Described therein is the use of oligo-aldimines, oligoketimines and hindered oligoamines to modify polyol compositions, leading to a reduction in the viscosities of the mixtures with retention and often improvement in network construction and in the physical properties. The publication, however, is concerned only with the addition of pigments to the modified polyol composition, and not with the production of highly filled modified polyol compositions.
It is known, it is true, that these modified coating compositions can comprise fillers such as calcium sulfate, zinc phosphate, and barium sulfate, for example, which exhibit very good adhesion to a steel substrate, for example (EP 686 654; Detlef Gysau, Technologie des Beschichtens—Füllstoffe, Vincentz Verlag, ISBN 3-87870-793-2). On aluminum and on multimaterial substrates, however, both in the loaded and unloaded states of the substrate, these coating compositions display unsatisfactory adhesion. Moreover, the weathering behavior of these coating compositions is unacceptable. Adhesion to aluminum is typically obtained by an alternative route, through the use of chromate-containing fillers (Lückert, Pigment+Füllstofftabellen, Lückert Verlag, ISBN 3-927342-03-3; Glasurithandbuch 1984, Vincentz Verlag, ISBN 3-87870-192-6; Peter Volk in “Metalloberfläche”, Sonderdruck May 2006). On account of the environmental pollution, however, the use of chromate for the purpose of achieving the desired properties ought to be abandoned.